The present invention relates to optical transducers (sometimes referred to as optical sensors) which utilize optical means for sensing mechanical displacements, such as movements of a body or deformations of a membrane, and converting them to electrical signals. The invention is particularly useful in optical microphones for converting sound (manifested by acoustically produced deformations of a membrane) into electrical signals, and is therefore described below with respect to this application.
Optical transducers of this type are described, for example, in U.S. Pat. Nos. 5,771,091; 5,969,838; 6,091,497; and 6,239,865, the contents of which patents are incorporated herein by reference.
Such optical transducers generally comprise a housing including a light source and a light detector laterally spaced from the light source; and a deformable membrane mounted to the housing over the light source and light detector and having an inner surface to reflect light from the light source to the light detector in accordance with the deformations of the membrane. The light detector thus produces an output electrical signal modulated in accordance with the deformations of the membrane.
The membranes in optical microphones function differently from membranes in other known types of microphones, such as condenser microphones, electret microphones, and electro-dynamic microphones. In a condenser microphone, the membrane is used as a capacitor plate for varying capacitance in accordance with the deformations of the membrane, and is therefore made of a metal or of a plastic having a metal layer. In electret microphones, the membrane is generally made of a plastic covered by an electret material that possesses a constant electrical charge in relation to the opposite capacitor plate. In electro-dynamic microphones, the membrane is generally made of a plastic carrying an electrical coil that moves in a magnetic field produced by an electromagnet to modulate the electrical output in accordance with the deformations of the membrane.
In the foregoing types of microphones, the complete area of the membrane is generally used in the conversion of mechanical movement of the membrane into the outputted electrical signal. For this reason, the sensitivity of the microphone can generally be increased by increasing the size of the membrane in these microphones.
Many applications of optical microphones do not permit large size membranes as commonly provided in capacitor, electret, or electro-dynamic microphones. Therefore, there is a need to provide optical microphones (or other types of optical transducers) with an increased sensitivity while still enabling the use of relatively small size membranes.